In the past, L-leucine have been produced by a method of fermentation primarily using microorganisms belonging to the genus Brevibacterium, Corynebacterium or Serratia or mutants thereof which produce L-leucine (Amino acid fermentation, JAPAN SCIENTIFIC SOCIETY'S PRESS, pp.397-422, 1986).
The highest level of L-leucine accumulation was obtained when using Brevibacterium flavum VKPM B2736: this strain produces L-leucine at a concentration up to 26 g/L on sucrose-containing media for 72 h of fermentation in a laboratory fermenter (USSR Author Certificate 1394711). And Brevibacterium lactofermentum 34 produces L-leucine up to 34 g/L on a medium with glucose (Appl. Environ. Microbiol., 51, p.1024 (1986)).
As described above, the productivity of L-leucine has been improved to some extent, however, the development of a more efficient and cost-effective method for producing L-leucine is required in order to meet increasing demand for L-leucine in the future.
On the other hand, microorganisms belonging to the genus Escherichia is potentially utilized as a potent L-leucine-producing bacteria due to its rapid growth rate, prominent data obtained from genetic analysis and plentiful genetic materials. However, there are few reports which disclose the production of L-leucine using bacteria belonging to the genus Escherichia.
As L-leucine-producing bacterial strains of the genus Escherichia, strain which is resistant to .beta.-thienylalanine, strain which is resistant to .beta.-thienylalanine and .beta.-hydroxyleucine (Japanese Patent Publication No.62-34397 for two) and a strain which is resistant to 4-azaleucine or 5,5,5-trifluoroleucine (Japanese Laid-Open Publication No. 8-70879) are known.
However, there has been known neither L-leucine-resistant bacteria belonging to the genus Escherichia nor a relation between L-leucine resistance and a productivity of L-leucine.